Hot-dip Zn—Al alloy coated steel sheets have been previously widely used as so-called precoated steel sheets having painted surfaces in fields of automobile, architecture, civil engineering, household electrical appliance, and the like. Hot-dip galvanized steel sheets having Al contents of 0.2 percent by mass or less in coating layers (hereafter referred to as GI), Galfan having an Al content of about 5 percent by mass in a coating layer (hereafter referred to as GF), and Galvalume steel sheets having Al contents of about 55 percent by mass in coating layers (hereafter referred to as GL) are mainly used as the hot-dip Zn—Al alloy coated steel sheet. In particular, in the field of architecture, civil, engineering, and the like, GF is used frequently on the ground that, for example, the cost is lower than the cost of GL and the corrosion resistance is superior to the corrosion resistance of GI.
However, GF has the following problems.
(i) Coating Appearance
Hexagonal patterned spangles are formed. The form of the spangle is different depending on coating conditions (for example, annealing before coating and components of a bath), cooling conditions after coating for example, cooling rate), and the like. Therefore, the appearance may be impaired in the case where the spangles are used without being covered. In the case where painting is performed and a color steel sheet is produced, spangles may come to a painting surface so as to impair the appearance after the painting. Consequently, in recent years, demands for GF having a beautiful coating layer with metallic luster and no spangle have increased.
(ii) Blackening Resistance
A so-called blackening phenomenon, in which a coating surface is discolored charcoal gray locally, may occur depending on a corrosive environment so as to impair a commercial value significantly. It is believed that the blackening occurs due to conversion of zinc oxide of the coating surface to oxygen-deficient zinc oxide in the case where the coating surface is placed in a high-temperature high-humidity environment or the like after coating. Relatively few problems occur in the case where a chemical conversion treatment and painting are performed just after coating. However, in many practical cases, packing is performed in the state of a coil after coating and the chemical conversion treatment and the painting arc performed after some period of time. Therefore, blackening occurs during the above-described period of time. In this case, the chemical conversion treatment may become faulty afterward. As a result, the adhesion of the painting film after the painting, the workability, the corrosion resistance, and the like may deteriorate and, thereby, the commercial value may be impaired significantly.
For the purpose of improving the blackening resistance and the like of the hot-dip Zn—Al alloy coated steel sheet having a GF composition, for example, the following proposals have been made previously.
Japanese Unexamined Patent Application Publication No. 2001-329354 discloses that more than 2 percent by mass to 10 percent by mass of Mg is added to a Zn—Al alloy coating layer containing 0.5 to 20 percent by mass of Al and the surface length factor of Zn—Al—Mg eutectic+Zn single phase of the coating surface is specified to be 50% or more for the purpose of improving the blackening resistance and the chemical conversion treatability. Furthermore, it is disclosed that at least one of Pb, Sn, Ni, and the like is added, if necessary, for the purpose of improving the chemical conversion treatability.
Japanese Unexamined Patent Application Publication No. 2003-183800 discloses that regarding, a chromate-treated hot-dip Zn—Al alloy coated steel sheet, 0.003 to 0.15 percent by mass of Ni and/or Ti is added to a Zn—Al alloy coating layer containing 2 to 15 percent by mass of Al, a chromate treatment is performed with a specific chromate treatment solution to allow concentrated Ni and/or Ti to present in an outermost surface portion of the coating layer, and the resulting Ni and/or Ti concentration portion and the interface of a chromate layer are integrated for the purpose of improving the blackening resistance and the corrosion resistance.
Japanese Unexamined Patent Application Publication No. 4-297562 discloses that regarding a Zn—Al alloy coating layer containing 4.0 to 7.0 percent by mass of Al, the Pb content is specified to be 0.01 percent by mass or less and the Sn content is specified to be 0.005 percent by mass or less, 0.005 to 3.0 percent by mass of Ni and 0.005 to 3.0 percent by mass of Cu are added, and a skin pass treatment and a chromate treatment are performed after the coating for the purpose of improving the blackening resistance.
Although the purpose is other than the improvement of the blackening resistance, Japanese Unexamined Patent Application Publication No. 2001-64759 discloses that 0.1 to 10 percent by mass of Mg is added to a Zn—Al alloy coating layer containing 0.1 to 40 percent by mass of Al so as to constitute a texture, in which Mg based intermetallic compound phases having a predetermined size are dispersed, for the purpose of improving the workability. Furthermore, it is disclosed that at least one of Ni, Ti, Sb, and the like is added, if necessary, for the purpose of improving the sliding resistance.
However, those disclosures continue to pose challenges.
Regarding the coated steel sheet of Japanese Unexamined Patent Application Publication No. 2001-329354, even when the blackening resistance can be improved to some extent, poor appearance of coating easily occurs due to degradation of color tone and dross adhesion. Furthermore, cracking easily occurs in the coating layer and, thereby, the workability easily deteriorates. If the Mg content increases, the blackening resistance also deteriorates.
Regarding the chromate-treated coated steel sheets of Japanese Unexamined Patent Application Publication Nos. 2003-183800 and 4-297562, the effect of improving the blackening resistance is unsatisfactory. Furthermore, poor appearance of a coated steel sheet or a painted steel sheet easily occurs because spangles are formed as in common GF. Regarding Japanese Unexamined Patent Application Publication No. 2003-183800, the chromate treatment by using a specific chromate treatment solution is required.
Regarding the coated steel sheet of Japanese Unexamined Patent Application Publication No. 2001-64759, one of problems, e.g., deterioration of the blackening resistance, poor appearance of coating due to degradation of color tone and dross adhesion, or poor appearance due to formation of spangles, occurs.
It could therefore be helpful to provide a hot-dip Zn—Al alloy coated steel sheet exhibiting a beautiful coating appearance with metallic luster, in which no spangle or very fine spangles are formed, and having excellent blackening resistance and a method for manufacturing the hot-dip Zn—Al alloy coated steel sheet.